Are Obagi Nu Derm system and hyaluronic acid compatible?

Glad you're having success. Even though it's called an acid, hyaluronic acid is a straight moisturizer - no pigment fading or other exfoliating properties and it can be used with NuDerm both morning and night. Vitamin C is a true acid (ascorbic acid) and depending on the strength, can be a mild exfoliant. It is also an antioxidant and if you use it in the morning before your sunscreen, it will help your sunscreen work better.

Adding a hyaluronic acid serum would likely be a nice addition to your Nu Derm routine. The HA serum should help to hydrate and moisturize the skin and I would apply it as you described, after your toner in the am and pm and before the other products in your routine. HQ is safe and effective when used properly and will generally tell my patients to take a couple months off their HQ and resume if their pigmentation starts to return. Adding a vitamin C during a break from HQ would be a nice addition and I would apply your Vitamin C in the morning. Good luck!

It is very important to use medical grade products prescribed by your physician. Hydroquinone products are to be used no longer then 4 months. Then you must stop using for months before starting again. There are several non hydroquinone products available that you can continuously use, such as ZO Medical Brightenex from Dr Zein Obagi's newest skin care line.
I would recommend scheduling a skin consult with your provider so the appropriate products are recommend for your skin.
Best,